This invention is concerned with the provision of a torque absorbing driving connection between a rotatable string of drill pipe of a rock drill and the rock bit.
The conventional string of drill pipe comprises lengths of drill pipe threadedly jointed to one another and to a rock bit at the bottom end of the string. During operation of the drill, the drill string is rotating under the power of a motor. The usual motor employed to rotate the drill string and the bit that the string carries does not have a high torque capability at a steady rate of operation but momentum developed by the motor plus the momentum of the rotating drill pipe during a drilling operation can cause a high torque pulse to be imparted to the bit and to the joints of the pipe comprising the drill string when rotation of the bit is momentarily stopped by impingement with the work. This undesirable high torque causes excessive tightening of the threaded joints connecting the sections of drill pipe and possible cracking or damage to the pipe itself. The excessive tightening of the drill pipe increases the usual difficulty later of loosening the pipe joints when withdrawing the drill string from the drill hole.
Accordingly, it is a general objective of this invention to provide means for absorbing this excessive torque so as to avoid the undesirable effects mentioned.
This objective is accomplished in the present invention by the provision of a torsionally elastic shaft connection between the drill string and the rock bit. The torsion shaft will normally suffice to absorb the excessive torque that occurs at times during a usual drilling operation. When, because of unusual circumstances, the bit stubbornly resists being rotated, a secondary drive means is provided to prevent overloading and further torquing of the torsion shaft. This secondary drive means is in the form of a clutch which responds to the latter situation to relieve the torsion shaft of further load and to transmit any added torque of the drill string directly to the rock bit.